There is an age-associated decline in sensitivity of peripheral tissues to insulin. Euglycemic hyperinsulinemic clamp studies have supported a postreceptor defect. As is typical of aging, there is a large variance in this insulin resistance, with some individuals performing comparably to younger groups. Caloric intake, body composition and physical activity are known to effect insulin action. Little attention has been paid to quantifying the effect of physical training on the insulin resistance of aging. The goals of this study are to assess the effect of baseline level of physical training on the insulin resistance of aging, and then to test the effect of a program of physical training in aged man. A group of healthy volunteers age 65 or older will be screened for safety of participation. Those qualifying without chronic diseases, obesity, or abnormal oral glucose tolerance will be characterized for level of physical training by determination of submaximal work capacity and maximal oxygen uptake (VO2 max). Body composition will be measured and caloric intake controlled. Each subject will undergo a 1 and 5 mU/kg/Min (in step) hyperinsulinemic euglycemic clamp with indirect calorimetry to determine insulin sensitivity, oxidative disposal rate of infused glucose, and insulin clearance rate. Intrinsic insulin secretion will be measured by a hyperglycemic clamp. Monocyte and erythrocyte insulin receptors will be measured. Total body glucose disposal (M) will be compared to submaximal work capacity and VO2 max in each individual by linear regression and multiple regression techniques to assess the effect of baseline level of physical training on insulin resistance. A group of 24 untrained subjects will then enter a 12 week supervised aerobic training program designed for achievement of 75% of maximum determined heart rate for 30 minutes, 3 days/week. Weight maintaining diets will be insured. Measurements of physical work capacity at a submaximal level will be performed every 4 weeks to adjust the level of training to increased work capacity. At the end of the training period, the initial parameters of submaximal work capacity and VO2 max, body composition, oral glucose tolerance test, hyperinsulinemic and hyperglycemic clamps, and insulin receptors will again be measured. Changes in M will be compared to Delta submaximal work capacity and Delta VO2 max using the same analytic techniques to assess the correlation of improved insulin sensitivity with improved level of physical training.